Extraretinal photoreception during the gonadal photorefractory period in the golden-crowned sparrow

Extraretinal photoreception is involved in the perception of light used to measure photoperiodic time during the initiation of gonadal growth in a number of birds. Evidence is presented which demonstrates that extraretinal photoreceptors are also involved in measuring photoperiodic time during the gonadal photorefractory period in the goldencrowned sparrow (Fig. 3). Untreated sparrows were able to terminate the refractory condition while being exposed to long dim days (16DL:8D; DL=0.2 lux). However, birds which had their head feathers clipped to allow more light to penetrate through to the brain were maintained in the refractory state under the same lighting conditions. These results demonstrate that extraretinal photoreception is involved in the maintenance of photorefractoriness in birds. It has been suggested that the eyes and extraretinal photoreceptors may both be involved in the initiation of gonadal growth in golden-crowned sparrows (Gwinner et al., 1971). This conclusion was based on the observation that a reduced rate of gonadal growth occurred in sparrows with shielded brains as compared to unshielded controls when both groups of birds were exposed to the same light treatment (i.e. 16L:8D; L=6 lux). The results presented here suggest that at a light intensity of 6 lux, light may have been reaching extraretinal photoreceptors even in birds with shielded brains. Therefore, the eyes may not be involved in testicular recrudescence in this species.